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

third_party/musl/rand48.h

@@ -0,0 +1,18 @@
+#ifndef COSMOPOLITAN_THIRD_PARTY_MUSL_RAND48_H_
+#define COSMOPOLITAN_THIRD_PARTY_MUSL_RAND48_H_
+#if !(__ASSEMBLER__ + __LINKER__ + 0)
+COSMOPOLITAN_C_START_
+
+double drand48(void);
+double erand48(unsigned short[3]);
+long int lrand48(void);
+long int nrand48(unsigned short[3]);
+long mrand48(void);
+long jrand48(unsigned short[3]);
+void srand48(long);
+unsigned short *seed48(unsigned short[3]);
+void lcong48(unsigned short[7]);
+
+COSMOPOLITAN_C_END_
+#endif /* !(__ASSEMBLER__ + __LINKER__ + 0) */
+#endif /* COSMOPOLITAN_THIRD_PARTY_MUSL_RAND48_H_ */