Function iscom 

pub fn iscom(s: &str) -> bool

Port of int iscom(char *s) from Src/exec.c:962.

C body:

struct stat statbuf;
char *us = unmeta(s);
return (access(us, X_OK) == 0 && stat(us, &statbuf) >= 0 &&
        S_ISREG(statbuf.st_mode));

True iff s names an executable regular file (X-perm + S_IFREG). Used by the PATH-search loop in findcmd / search_defpath to validate candidate paths before exec.