Speed up 'read', fixing macOS hang

This fixes a hanging bug that could occur on macOS when using the
'read' command to read from a FIFO and encountering end-of-file
without a final newline character. It also makes the 'read' command
perform 15-25% faster on macOS and Linux and maybe other systems.

src/lib/libast/sfio/sfpkrd.c: sfpkrd():
- Get rid of the optional stuff that uses the poll(2) or select(2)
  syscalls. The only thing that is required to avoid regressions is
  the code that was conditional upon the socket_peek feature test,
  which tests for the correct functioning of the recv(2) syscall.
  This has now been made mandatory. The rest now uses what was
  previously a fallback in plain C, resulting in a function that is
  not only more readable, but actually faster than the syscalls.

Resolves: https://github.com/ksh93/ksh/issues/118

NEWS

@@ -3,6 +3,11 @@ For full details, see the git log at: https://github.com/ksh93/ksh
 
 Any uppercase BUG_* names are modernish shell bug IDs.
 
+2020-08-18:
+
+- Sped up the 'read' command on most systems by 15-25%. Fixed a hanging bug
+  on reading from a FIFO that could occur on macOS.
+
 2020-08-17:
 
 - 'command -p' incorrectly used the hash table entry (a.k.a. tracked alias)