Add three options to 'ulimit' (#406)

This patch adds a few extra options to the ulimit command (if the OS
supports them). These options are also present in Bash, although in ksh
additional long forms of each option are available:
  ulimit -k/--kqueues   This is the maximum number of kqueues.
  ulimit -P/--npts      This is the maximum number of pseudo-terminals.
  ulimit -R/--rttime    This is the time a real-time process can run
                        before blocking, in microseconds. When the
                        limit is exceeded, the process is sent SIGXCPU.

Other changes:
- bltins/ulimit.c: Change the formatting from sfprintf and increase the
  size of the tmp buffer to prevent text from being cut off in ulimit
  -a (this was required to add ulimit -R).
- data/limits.c: Add support for using microseconds as a unit.

NEWS

@@ -3,6 +3,17 @@ For full details, see the git log at: https://github.com/ksh93/ksh
 
 Any uppercase BUG_* names are modernish shell bug IDs.
 
+2021-12-28:
+
+- Added three options to the ulimit builtin with the same names and
+  functionality as in Bash:
+  - 'ulimit -k' sets the maximum number of kqueues.
+  - 'ulimit -P' sets the maximum number of pseudo-terminals.
+  - 'ulimit -R' sets the maximum time in microseconds a real-time process
+    can run before blocking.
+  Note that to use these options the operating system must support the
+  corresponding resource limit.
+
 2021-12-27:
 
 - Two bash-like flags for 'whence' were backported from ksh 93v-: