sleep: code shrink

sleep: code shrink

Use less stack by using same "sigset_t set" object for new and saved
signal set, remove redundant clearing of set, and do not save/restore
errno around sigprocmask(SIG_SETMASK) - it never changes it.

While at it, improve comments and make code style consistent
across sleep.c file.

    text           data     bss     dec     hex filename
-    242              0       0     242      f2 libc/unistd/sleep.o
+    197              0       0     197      c5 libc/unistd/sleep.o

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>