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`

libc/nt/kernel32/DeleteFileW.s

@@ -2,7 +2,7 @@
 .imp	kernel32,__imp_DeleteFileW,DeleteFileW,0
 
 	.text.windows
-DeleteFile:
+__DeleteFile:
 	push	%rbp
 	mov	%rsp,%rbp
 	.profilable
@@ -11,5 +11,5 @@ DeleteFile:
 	call	*__imp_DeleteFileW(%rip)
 	leave
 	ret
-	.endfn	DeleteFile,globl
+	.endfn	__DeleteFile,globl
 	.previous

libc/nt/kernel32/RemoveDirectoryW.s

@@ -2,7 +2,7 @@
 .imp	kernel32,__imp_RemoveDirectoryW,RemoveDirectoryW,0
 
 	.text.windows
-RemoveDirectory:
+__RemoveDirectory:
 	push	%rbp
 	mov	%rsp,%rbp
 	.profilable
@@ -11,5 +11,5 @@ RemoveDirectory:
 	call	*__imp_RemoveDirectoryW(%rip)
 	leave
 	ret
-	.endfn	RemoveDirectory,globl
+	.endfn	__RemoveDirectory,globl
 	.previous

libc/nt/kernel32/SetCurrentDirectoryW.s

@@ -2,7 +2,7 @@
 .imp	kernel32,__imp_SetCurrentDirectoryW,SetCurrentDirectoryW,0
 
 	.text.windows
-SetCurrentDirectory:
+__SetCurrentDirectory:
 	push	%rbp
 	mov	%rsp,%rbp
 	.profilable
@@ -11,5 +11,5 @@ SetCurrentDirectory:
 	call	*__imp_SetCurrentDirectoryW(%rip)
 	leave
 	ret
-	.endfn	SetCurrentDirectory,globl
+	.endfn	__SetCurrentDirectory,globl
 	.previous