Minor documentation and script cleanups (#6868)

* Remove mention of no longer used IRC network

Channels exist on matrix, and on libera.chat now.

* Correctly match only files with .png extension

The current search would match files such as 'createpng'.

* Fix comparison in script

The result was always false, due to comparing a literal string instead of a variable.

* Use correct license files from upstream

Correct license files obtained from:
https://www.gnu.org/licenses/old-licenses/gpl-2.0.txt
https://www.gnu.org/licenses/gpl-3.0.txt
https://www.gnu.org/licenses/old-licenses/lgpl-2.1.txt
https://www.gnu.org/licenses/lgpl-3.0.txt

* Refresh several shell scripts

This fixes several shellcheck warnings, as well as makes the code more
robust and have consistent codestyle between all the files.

* Trim excess whitespace

cmake/CodeCoverage.cmake

@@ -70,7 +70,7 @@ include(CMakeParseArguments)
 
 # Check prereqs
 find_program( GCOV_PATH gcov )
-find_program( LCOV_PATH  NAMES lcov lcov.bat lcov.exe lcov.perl)
+find_program( LCOV_PATH NAMES lcov lcov.bat lcov.exe lcov.perl)
 find_program( GENHTML_PATH NAMES genhtml genhtml.perl genhtml.bat )
 find_program( GCOVR_PATH gcovr PATHS ${CMAKE_SOURCE_DIR}/scripts/test)
 find_program( SIMPLE_PYTHON_EXECUTABLE python )
@@ -300,4 +300,4 @@ function(APPEND_COVERAGE_COMPILER_FLAGS)
     set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} ${COVERAGE_COMPILER_FLAGS}" PARENT_SCOPE)
     set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${COVERAGE_COMPILER_FLAGS}" PARENT_SCOPE)
     message(STATUS "Appending code coverage compiler flags: ${COVERAGE_COMPILER_FLAGS}")
-endfunction() # APPEND_COVERAGE_COMPILER_FLAGS
+endfunction() # APPEND_COVERAGE_COMPILER_FLAGS

cmake/FindReadline.cmake

@@ -47,4 +47,3 @@ mark_as_advanced(
     Readline_INCLUDE_DIR
     Readline_LIBRARY
 )
-