Fix reserved identifier violations.

Some identifiers [do not fit](https://www.securecoding.cert.org/confluence/display/cplusplus/DCL32-CPP.+Do+not+declare+or+define+a+reserved+identifier "Do not use identifiers that are reserved for the compiler implementation.") to the expected naming convention for the programming language "C". They can be renamed.

Is this update suggestion more acceptable than [the previous approach](https://github.com/cmus/cmus/pull/183 "Renaming of some identifiers")?