Add a argument to specify the name of the target function

[rafaelsamenezes]

Is your feature request related to a problem? Please describe.
FuseBMC is trying to use map2check to create coverage for C programs. However, its targets consists in functions named GOAL_N (where N is a number).

Describe the solution you'd like
Add the option --target-function-name which will set the target function name for map2check. This will require patches over the TargetPass and over the opt calls.

Describe alternatives you've considered
The tool that is using map2check could instead rename the current target into __VERIFIER_error()

Additional context
Some reference:

• CommandLine: http://llvm.org/docs/CommandLine.html
• This answer: https://stackoverflow.com/a/13668358