housekeeping changes

[bill-auger]

i have been putting these together for some months now - none
of these changes affect the functionality of makeicecat or modify
the icecat sources; but they are intrusive and would be tedious
to separate into individually applicable patches - these are under
source control now, based on top of the current gnuzilla 'master'
branch - i could re-work/tease-out any that are not desirable later
- i have compiled the generated icecat with these changes for v78.4
and v78.6 - those builds have been in use by parabola users for
about 3 weeks; so i am reasonably confident with these changes now

  $ git remote add parabola https://git.parabola.nu/~bill-auger/icecat.git
  $ git fetch parabola
  $ git checkout parabola/v78.6


a brief explanation of each change:

9710e2d  refactor into a stack of discrete functions
  - self-explanitory - this was a feature request from months ago
    basically, just refactoring linear "### section comments ###" into functions

0bfbd8d  add switch for development features
  - this does not do much yet -  but to guard anything which is only interesting
    for developement/debugging tasks - eg: verbose logging, early exits, `set 
-x`, etc

d678974  allow rebuilds - keep orig sources - cleanup work dir
  - this preserves/restores some source directories so they do not need to be 
downloaded
    every time makeicecat is run - i have some WIP ideas to make the sources 
reproducible,
    eg: caching/pinning VCS checkout-IDs, or downloading .xpi langpacks  
instead of from VCS

d541ad0  more environment sanity checks
  - more cross-distro support like 'rename' below

081d86a  allow rename command to be detected at runtime
  - this is to abstract-away cross-distro differences in the 'rename' program

01801fe  housekeeping - delete unused 'sedhelper2()' function
  - self-explanitory (it was not used)

128e863  housekeeping - normalize variables
  - consistent naming for variables

25264dc  housekeeping - extract and normalize config constants
  - puts the commonly changed URLS/checksums, and other magic-numbers, at the 
top of the script

69768a7  add output dir to .gitignore
  - self-explanitory - this helps with the later 'allow rebuilds' change