plugins: reorder source files to ease future diffs

We put main.c at the end of the list to each plugin can be added
alphabetically, without always changing the last backlash hanling
plugins: adding an external_ wildcard plugin
plugins: enhance code readbility in "listplugins"
plugins: use strlen() for readability
plugins: external_ is a wildcard plugin
p/external_: fetch is only default

"fetch" was overrided everytime, no it is only a default, and
overrided when needed
p/external_: read the correct env var
Merge branch 'external_' into cygwin