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.

src/cmd/ksh93/include/ulimit.h

@@ -103,6 +103,9 @@
 #ifndef RLIMIT_FSIZE
 #define RLIMIT_FSIZE	RLIMIT_UNKNOWN
 #endif
+#ifndef RLIMIT_KQUEUES
+#define RLIMIT_KQUEUES	RLIMIT_UNKNOWN
+#endif
 #ifndef RLIMIT_LOCKS
 #define RLIMIT_LOCKS	RLIMIT_UNKNOWN
 #endif
@@ -121,6 +124,9 @@
 #ifndef RLIMIT_NPROC
 #define RLIMIT_NPROC	RLIMIT_UNKNOWN
 #endif
+#ifndef RLIMIT_NPTS
+#define RLIMIT_NPTS	RLIMIT_UNKNOWN
+#endif
 #ifndef RLIMIT_PIPE
 #define RLIMIT_PIPE	RLIMIT_UNKNOWN
 #endif
@@ -133,6 +139,9 @@
 #ifndef RLIMIT_RTPRIO
 #define RLIMIT_RTPRIO	RLIMIT_UNKNOWN
 #endif
+#ifndef RLIMIT_RTTIME
+#define RLIMIT_RTTIME	RLIMIT_UNKNOWN
+#endif
 #ifndef RLIMIT_SBSIZE
 #define RLIMIT_SBSIZE	RLIMIT_UNKNOWN
 #endif
@@ -154,6 +163,7 @@
 #define LIM_BYTE	2
 #define LIM_KBYTE	3
 #define LIM_SECOND	4
+#define LIM_MICROSECOND	5
 
 typedef struct Limit_s
 {