fix bug in path logic gef-extras.sh

[therealdreg]

This PR improves and fix a bug in path logic (introduced in #870) (gef-extras.sh)

Now it works with named args (@hugsy suggestion):

./gef-extras.sh -h
Usage: /home/dreg/gef-extras.sh [-b <main|dev>] [-p <path_to_install>]

By default it uses main and default path logic

Examples of use:

./gef-extras.sh <- (main branch, default path)
./gef-extras.sh -b main <- (main branch, default path)
./gef-extras.sh -b dev <- (dev branch, default path)
./gef-extras.sh -b dev -p /home/kali/gextras <- (dev branch, specific path)
./gef-extras.sh -p /home/kali/gextras <- (main branch, specific path)
./gef-extras.sh -b main -p /home/kali/gextras <- (main branch, specific path)

Against which architecture was this tested ?

"Tested" indicates that the PR works and the unit test (i.e. make test) run passes without issue.

• x86-32
• x86-64
• ARM
• AARCH64
• MIPS
• POWERPC
• SPARC
• RISC-V

Checklist

• My PR was done against the dev branch, not main.
• My code follows the code style of this project.
• My change includes a change to the documentation, if required.
• If my change adds new code, adequate tests have been added.
• I have read and agree to the CONTRIBUTING document.