gdb: make skip_over_slash_fmt available outside printcmd.c

gdb: make skip_over_slash_fmt available outside printcmd.c

Move the function skip_over_slash_fmt into completer.c, and make it
extern, with a declaration in completer.h.

This is a refactor in order to support the next commit.  I've not
changed any of the code in skip_over_slash_fmt.

There should be no user visible changes after this commit.