Add casts to fix strict compiler errors [-fpermissive]

When using g++ and enforcing strict compiler errors, this commit fixes the following:

mongoose.c: In function ‘char* mg_fgets(char*, size_t, file*, char**)’:
mongoose.c:2405:60: error: invalid conversion from ‘void*’ to ‘char*’ [-fpermissive]

mongoose.c: In function ‘int set_ports_option(mg_context*)’:
mongoose.c:4586:64: error: invalid conversion from ‘void*’ to ‘socket*’ [-fpermissive]

mongoose.c: In function ‘void* worker_thread(void*)’:
mongoose.c:5104:28: error: invalid conversion from ‘void*’ to ‘mg_context*’ [-fpermissive]

mongoose.c: In function ‘void* master_thread(void*)’:
mongoose.c:5220:28: error: invalid conversion from ‘void*’ to ‘mg_context*’ [-fpermissive]
mongoose.c:5235:58: error: invalid conversion from ‘void*’ to ‘pollfd*’ [-fpermissive]