Optimize program flow for execution speed. Also fix some more style(9) nits
while here:
+ Fix an issue when extracting small archives where dialog_mixedgauge was
  not rendering; leaving the user wondering if anything happened.
+ Add #ifdef's to assuage compilation against older libarchive
  NB: Minimize diff between branches; make merging easier.
+ Add missing calls to end_dialog(3)
+ Change string processing from strtok(3) to strcspn(3) (O(1) optimization)
+ Use EXIT_SUCCESS and EXIT_FAILURE instead of 0/1
+ Optimize getenv(3) use, using stored results instead of calling repeatedly
  NB: Fixes copy/paste error wherein we display getenv(BSDINSTALL_DISTDIR) in
      an error msgbox when chdir(2) to getenv(BSDINSTALL_CHROOT) fails
      (wrong variable displayed in msgbox).
+ Use strtol(3) instead of [deprecated] atoi(3)
+ Add additional error checking (e.g., check return of archive_read_new(3))
+ Assign DECONST strings to static variables
+ Fix typo in distextract.c error message (s/Could could/Could not/)
+ Add comments and make a minor whitespace adjustment

Reviewed by:	nwhitehorn, julian