posix/bits/unistd.h: Clearly separate declaration from definitions

This change is similar to what was done for bits/wchar2.h.
Routines declaration are moved into a dedicated bits/unistd-decl.h file
which is then included into the bits/unistd.h file.

This will allow to adapt the files so that PLT entries are not created when
_FORTIFY_SOURCE is enabled.

Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>