bug#32746: stop using obsolescent setitimer API

[Paul Eggert]

POSIX obsoleted the getitimer/setitimer API a decade ago, and now's a good time 
for Emacs to stop using this obsolescent API, partly to simplify other timestamp 
improvements that I have in mind, partly to head off any other lurking bugs like 
the problem I just fixed in commit 238c7cd730819ddba2dbde3c46ee36136575695b 
relating to a mistaken assumption about the old API.

Attached please find two proposed patches. The first simplifies Emacs proper to 
just use the current timer_gettime/timer_settime API instead (this is the 
standard replacement for the obsolescent API). The second patch affects only the 
MS-Windows code, modifying it to use the new API instead of the old one; I 
haven't tested it as I don't use MS-Windows.

0001-Simplify-by-not-worrying-about-setitimer.patch
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0002-Adjust-w32-to-emulate-timer_settime-not-setitimer.patch
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