gdb/gdbserver: move argument setting complexity into gdbsupport/

gdb/gdbserver: move argument setting complexity into gdbsupport/

This commit removes some uses of gdb::argv_vec from GDB.

The gdb::argv_vec class exists in part due to our need to convert from
one container type to another in a few places, and I think by using
templates we can push this conversion down into
gdbsupport/common-inferior.{cc,h} and remove the conversion logic from
higher level code in GDB.  This should make core GDB simpler.

For examples of this simplification, see python/py-inferior.c,
remote.c, and unittests/remote-arg-selftests.c, where this commit has
allowed for the removal of some code that only exists in order to
convert the container type.

Ideally I'd like to see all uses of gdb::argv_vec removed, but I'm
still not sure about the use in nat/fork-inferior.c, I think its use
there might be the best solution, so for now at least, I have no plans
to touch that code.

There should be no user visible changes after this commit.

Approved-By: Tom Tromey <tom@tromey.com>