musllibc: fix restrict violation warnings

Cherry-picking commit e0eee3ceefd550724058ffbdf878e9eb06e18f18 from
musllibc mainline. Original comment: The old/new parameters to
pthread_sigmask, sigprocmask, and setitimer are marked restrict, so
passing the same address to both is prohibited. Modify callers of
these functions to use a separate object for each argument.

Signed-off-by: Axel Heider <axelheider@gmx.de>

use 0 instead of NULL for null pointer constants

and thereby remove otherwise-unnecessary inclusion of stddef.h

include cleanups: remove unused headers and add feature test macros

initial check-in, version 0.5.0