Improve cosmo's conformance to libc-test

This change addresses various open source compatibility issues, so that
we pass 313/411 of the tests in https://github.com/jart/libc-test where
earlier today we were passing about 30/411 of them, due to header toil.
Please note that Glibc only passes 341/411 so 313 today is pretty good!

- Make the conformance of libc/isystem/ headers nearly perfect
- Import more of the remaining math library routines from Musl
- Fix inconsistencies with type signatures of calls like umask
- Write tests for getpriority/setpriority which work great now
- conform to `struct sockaddr *` on remaining socket functions
- Import a bunch of uninteresting stdlib functions e.g. rand48
- Introduce readdir_r, scandir, pthread_kill, sigsetjmp, etc..

Follow the instructions in our `tool/scripts/cosmocc` toolchain to run
these tests yourself. You use `make CC=cosmocc` on the test repository

libc/runtime/runtime.h

@@ -7,6 +7,7 @@ COSMOPOLITAN_C_START_
 ╚────────────────────────────────────────────────────────────────────────────│*/
 
 typedef long jmp_buf[8];
+typedef long sigjmp_buf[12];
 
 extern char **environ;                      /* CRT */
 extern int __argc;                          /* CRT */
@@ -49,6 +50,7 @@ extern size_t __virtualmax;
 extern bool __isworker;
 
 void mcount(void);
+int daemon(int, int);
 int _freestack(void *);
 void _bt(const char *, ...);
 unsigned long getauxval(unsigned long);
@@ -60,6 +62,8 @@ void longjmp(jmp_buf, int) libcesque wontreturn paramsnonnull();
 axdx_t setlongerjmp(jmp_buf) libcesque returnstwice paramsnonnull();
 void longerjmp(jmp_buf, intptr_t) libcesque wontreturn paramsnonnull();
 int _setjmp(jmp_buf) libcesque returnstwice paramsnonnull();
+int sigsetjmp(sigjmp_buf, int) libcesque returnstwice paramsnonnull();
+void siglongjmp(sigjmp_buf, int) libcesque wontreturn paramsnonnull();
 void _longjmp(jmp_buf, int) libcesque wontreturn paramsnonnull();
 void exit(int) wontreturn;
 void _exit(int) libcesque wontreturn;