basic/stat-util: avoid access syscall

basic/stat-util: avoid access syscall

I was looking at strace for systemd-getty-generator and noticed the call to
faccessat2(3</sys>, "", W_OK, AT_EMPTY_PATH), even though we already did
fstatfs(3</sys>), which should give us all the necessary information. Let's
only do this additional check when it's likely to yield something useful, i.e.
for network fses and otherwise skip the syscall.

The call to statvfs is replaced by statfs because that gives us the .f_type
field and allows is_network_fs() to be called.

I'm a bit worried that the is_network_fs() is somewhat costly. This will be
improved in later commits.