instrument functions: Use an environment variable instead of config files

instrument functions: Use an environment variable instead of config files

If the environment variable INSTRUMENT is
- unset or set to an empty string, print nothing, like with no
  instrumentation
- set to "all" or "a", print all the functions names
- set to "global" or "g", print only the global functions names

The configuration with --enable-instrument-functions remains.

Note that before this change, the default was to print all functions.
Now it is to print nothing. So by default 'make check' runs without errors.

This allows to run:
$ INSTRUMENT=a ./tcpdump ...
$ INSTRUMENT=g ./tcpdump ...
$ INSTRUMENT= ./tcpdump ...
or
$ export INSTRUMENT=global
$ ./tcpdump ...

This also allows to run the statically compiled binary on another host
after copying it.

It is no longer necessary to modify the configuration with:
$ make instrument_all
$ make instrument_global
$ make instrument_off
(Targets removed.)

Update .gitignore, CONTRIBUTING.md and Makefile.in accordingly.

Moreover:
Reduce the scope of a variable.
Rename a variable.
Remove '\n' in the perror() call.
Remove 2 spaces in function calls (style).

[skip ci]