patch 9.0.1544: recent glibc marks sigset() as a deprecated

Problem:    Recent glibc marks sigset() as a deprecated.
Solution:   Use sigaction() in mch_signal() if possible. (Ozaki Kiichi,
            closes #12373)

src/proto/os_unix.pro

@@ -1,4 +1,5 @@
 /* os_unix.c */
+sighandler_T mch_signal(int sig, sighandler_T func);
 int mch_chdir(char *path);
 void mch_write(char_u *s, int len);
 int mch_inchar(char_u *buf, int maxlen, long wtime, int tb_change_cnt);