BUILD: vars: avoid overlapping field initialization

BUILD: vars: avoid overlapping field initialization

Compiling vars.c with gcc 4.2 shows that we're initializing some local
structs field members in a not really portable way:

src/vars.c: In function 'vars_parse_cli_set_var':
src/vars.c:1195: warning: initialized field overwritten
src/vars.c:1195: warning: (near initialization for 'px.conf.args')
src/vars.c:1195: warning: initialized field overwritten
src/vars.c:1195: warning: (near initialization for 'px.conf')
src/vars.c:1201: warning: initialized field overwritten
src/vars.c:1201: warning: (near initialization for 'rule.conf')

It's totally harmless anyway, but better clean this up.