scripts: minor refactor to scripts

[arvindbr8]

Fixing parts of https://github.com/grpc/grpc-go/issues/7400.

Changes:

• install_protoc.sh
  • rename to install-protoc.sh ( following the rest of the file names under scripts/ )
  • In order to easily find the start of the program, put the main program in a function called main as the bottom-most function.
  • Move all checks from download_binary() to main()
  • Modify top-level comment to talk about usage of the script
  • Remove INSTALL_PATH=${1:+"$1"}
• regenerate.sh
  • exec and not source install_protoc.sh as the very first thing in the script
  • export GOBIN
  • does not save and restore $PATH anymore
• vet_common.sh
  • rename to common.sh

This PR does not make functional changes to any scripts -- which will be made in an upcoming PR

RELEASE NOTES: none

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 81.51%. Comparing base (e54f441) to head (c103912). Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7403      +/-   ##
==========================================
+ Coverage   81.46%   81.51%   +0.05%     
==========================================
  Files         348      348              
  Lines       26752    26752              
==========================================
+ Hits        21793    21807      +14     
+ Misses       3771     3764       -7     
+ Partials     1188     1181       -7     

see 17 files with indirect coverage changes