path helpers: move git_mkstemp* to wrapper.c

path helpers: move git_mkstemp* to wrapper.c

git_mkstemp_mode and related functions do not require access to
specialized git machinery, unlike some other functions from
path.c (like set_shared_perm()).  Move them to wrapper.c where
the wrapper xmkstemp_mode is defined.

This eliminates a dependency of wrapper.o on environment.o via
path.o.  With typical linkers (e.g., gcc), that dependency makes
programs that use functions from wrapper.o and not environment.o
or path.o larger than they need to be.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>