Fix some formatting errors, typos and other problems (#78)

Some notes:
- Removed a TODO note that was fixed in commit 43d9fbac.
- Removed a duplicate note about the '%l' time format in the changelog.
- Applied the following documentation fixes from Terrence J. Doyle:
  - https://www.mail-archive.com/ast-developers@lists.research.att.com/msg01852.html
  - https://www.mail-archive.com/ast-developers@lists.research.att.com/msg01856.html
- Fixed strange grammar in one of the error messages.
- Added missing options for rksh to the synopsis section.
- Applied a formatting fix from ksh93v- to the man page.
- Replaced a C99 line comment in src/lib/libast/comp/realpath.c with a
  proper comment that is valid in C89.
- Prioritize UTC over GMT in the documentation (missed by commit c9634e90).
- Add some extra information for 'ksh -R file' to the man page. This patch
  is from Red Hat: https://git.centos.org/rpms/ksh/blob/c8/f/SOURCES/ksh-20080202-manfix.patch

src/lib/libcmd/cp.c

@@ -61,7 +61,7 @@ static const char usage_cp[] =
 "[L:logical|dereference?Follow symbolic links and copy the files they "
     "point to.]"
 "[P|d:physical|nodereference?Don't follow symbolic links; copy symbolic "
-    "rather than the files they point to.]"
+    "links rather than the files they point to.]"
 ;
 
 static const char usage_ln[] =

src/lib/libcmd/date.c

@@ -151,7 +151,7 @@ USAGE_LICENSE
 "			\"%?%l\"; export TM_OPTIONS=\"format='\aoverride\a'\""
 "			to override the default]"
 "}"
-"[i:incremental|adjust?Set the system time in incrementatl adjustments to"
+"[i:incremental|adjust?Set the system time in incremental adjustments to"
 "	avoid complete time shift shock. Negative adjustments still maintain"
 "	monotonic increasing time. Not available on all systems.]"
 "[L:last?List only the last time for multiple \adate\a operands.]"