Fix inconsistent usage of various bool types

Fix inconsistent usage of various bool types
Convert u_charsToUChars_safe to ICU style error handling
Unify error handling code across functions to use the same patterns
Add static modifier to functions that are used only from single C file

Commit migrated from https://github.com/dotnet/coreclr/commit/c152ba92901a3345a58a42f2a4c5ac5634cfbd3e