Rely on <unistd.h> to declare 'environ',

Rely on <unistd.h> to declare 'environ',

as gnulib does this if the system doesn't.
* lib-src/emacsclient.c (environ): Remove decl.
* src/callproc.c, src/editfns.c, src/process.c (environ) [!USE_CRT_DLL]:
* src/emacs.c (environ) [DOUG_LEA_MALLOC]:
* src/vm-limit.c (environ) [ORDINARY_LINK]: Remove decl.
* src/vm-limit.c: Include <unistd.h>, for 'environ'.