Cleaned up variables extension.

src/lib-sieve/plugins/variables/cmd-set.c

src/lib-sieve/plugins/variables/ext-variables-arguments.c

src/lib-sieve/plugins/variables/ext-variables-arguments.h

src/lib-sieve/plugins/variables/ext-variables-common.c

src/lib-sieve/plugins/variables/ext-variables-common.h

src/lib-sieve/plugins/variables/ext-variables-limits.h

src/lib-sieve/plugins/variables/ext-variables-modifiers.c

src/lib-sieve/plugins/variables/ext-variables-modifiers.h

src/lib-sieve/plugins/variables/ext-variables-name.c

src/lib-sieve/plugins/variables/ext-variables-name.h

src/lib-sieve/plugins/variables/ext-variables-operands.c

src/lib-sieve/plugins/variables/ext-variables-operands.h

src/lib-sieve/plugins/variables/ext-variables.c

src/lib-sieve/plugins/variables/sieve-ext-variables.h

src/lib-sieve/plugins/variables/tst-string.c