Improve signals and memory protection

- Document sigaction()
- Simplify New Technology fork() code
- Testing and many bug fixes for mprotect()
- Distribute Intel Xed ILD in the amalgamation
- Turn Xed enums into defines to avoid DWARF bloat
- Improve polyfilling of SA_SIGINFO on BSDs and fix bugs
- setpgid(getpid(), getpid()) on Windows will ignore CTRL-C
- Work around issues relating to NT mappings being executable
- Permit automatic executable stack override via `ape_stack_pf`

Makefile

@@ -294,6 +294,7 @@ COSMOPOLITAN_OBJECTS =		\
 	THIRD_PARTY_COMPILER_RT	\
 	LIBC_THREAD		\
 	LIBC_TINYMATH		\
+	THIRD_PARTY_XED		\
 	LIBC_STR		\
 	LIBC_SYSV		\
 	LIBC_INTRIN		\
@@ -317,6 +318,7 @@ COSMOPOLITAN_HEADERS =		\
 	LIBC_RUNTIME		\
 	LIBC_SOCK		\
 	LIBC_STDIO		\
+	THIRD_PARTY_XED		\
 	LIBC_STR		\
 	LIBC_SYSV		\
 	LIBC_THREAD		\