Bug#952927: [live-build] inconsistency and unnecessary complexity in bin checking

[jnqnfe@gmail.com]

Package: live-build
Version: 1:20191221
Owner: jnqnfe@gmail.com
Severity: minor

there are lots of checks being done that tools are available and
executable. this is done with a mixture of use of `which` and fixed
paths, redirection of output to /dev/null (`2>/dev/null`) and redundant
executability tetsing on top of `which` checks (which already does such
a check).

this can be tidied up and robustified.
 - we can move everything to `which` and drop fixed paths, thus
robustifying things should a bin every move (you never know).
 - we can drop performing pointless `-x` testig on top of `which` use,
since `which` already performs this and signals such in its exist code
(returns 1 if nonexistant or nonexecutable).
 - we can drop the `2>/dev/null` redirection which in my testing seemed
to make no difference.

proposal: switch to conditionals of the form:
```
if [ $(which dpkg) ]; then
    #whatever
fi
```

patch to be submitted via salsa soon