tree-wide: make sure net/if.h is included before any linux/ header

tree-wide: make sure net/if.h is included before any linux/ header

The linux/ headers include linux/libc-compat.h that makes sure the
linux/ headers won't redeclare symbols already declared by net/if.h, but
glibc's net/if.h doesn't do that, so if the include order is reversed
we'll end up with a bunch of errors about redeclared stuff:

[3/519] Compiling C object test-network-tables.p/src_network_test-network-tables.c.o
FAILED: test-network-tables.p/src_network_test-network-tables.c.o
cc -Itest-network-tables.p -I. -I.. -Isrc/basic -I../src/basic -Isrc/fundamental -I../src/fundamental -Isrc/systemd -I../src/systemd -I../src/libsystemd/sd-bus -I../src/libsystemd/sd-device -I../src/libsystemd/sd-event -I../src/libsystemd/sd-hwdb -I../src/libsystemd/sd-id128 -I../src/libsystemd/sd-journal -I../src/libsystemd/sd-netlink -I../src/libsystemd/sd-network -I../src/libsystemd/sd-resolve -Isrc/shared -I../src/shared -Isrc/libsystemd-network -I../src/libsystemd-network -Isrc/network -I../src/network -I../src/network/netdev -I../src/network/tc -fdiagnostics-color=always -D_FILE_OFFSET_BITS=64 -Wall -Winvalid-pch -Wextra -std=gnu11 -O0 -g -Wno-missing-field-initializers -Wno-unused-parameter -Wno-nonnull-compare -Warray-bounds -Warray-bounds=2 -Wdate-time -Wendif-labels -Werror=format=2 -Werror=format-signedness -Werror=implicit-function-declaration -Werror=implicit-int -Werror=incompatible-pointer-types -Werror=int-conversion -Werror=missing-declarations -Werror=missing-prototypes -Werror=overflow -Werror=override-init -Werror=return-type -Werror=shift-count-overflow -Werror=shift-overflow=2 -Werror=strict-flex-arrays -Werror=undef -Wfloat-equal -Wimplicit-fallthrough=5 -Winit-self -Wlogical-op -Wmissing-include-dirs -Wmissing-noreturn -Wnested-externs -Wold-style-definition -Wpointer-arith -Wredundant-decls -Wshadow -Wstrict-aliasing=2 -Wstrict-prototypes -Wsuggest-attribute=noreturn -Wunused-function -Wwrite-strings -Wzero-length-bounds -fdiagnostics-show-option -fno-common -fstack-protector -fstack-protector-strong -fstrict-flex-arrays --param=ssp-buffer-size=4 -Wno-unused-result -Werror=shadow -fno-strict-aliasing -fstrict-flex-arrays=1 -fvisibility=hidden -fno-omit-frame-pointer -include config.h -pthread -DTEST_CODE=1 -MD -MQ test-network-tables.p/src_network_test-network-tables.c.o -MF test-network-tables.p/src_network_test-network-tables.c.o.d -o test-network-tables.p/src_network_test-network-tables.c.o -c ../src/network/test-network-tables.c
In file included from ../src/basic/linux/if_bonding.h:47,
                 from ../src/network/netdev/bond.h:5,
                 from ../src/network/test-network-tables.c:3:
../src/basic/linux/if.h:111:41: error: redeclaration of enumerator ‘IFF_UP’
  111 | #define IFF_UP                          IFF_UP
      |                                         ^~~~~~
../src/basic/linux/if.h:84:9: note: previous definition of ‘IFF_UP’ with type ‘enum net_device_flags’
   84 |         IFF_UP                          = 1<<0,  /* sysfs */
      |         ^~~~~~
../src/basic/linux/if.h:112:41: error: redeclaration of enumerator ‘IFF_BROADCAST’
  112 | #define IFF_BROADCAST                   IFF_BROADCAST
      |                                         ^~~~~~~~~~~~~
...

This also drops remaining workarounds from the last time this issue was
brought up (6f270e6bd8) since they shouldn't be needed anymore if the
order of the includes is the "correct" one. I also added a comment to
each affected include when this is inevitably encountered again in the
future.

Resolves: #32160