Call xsetenv() in main process instead of child

This makes xsetenv internal to x.c, and allows iso14755's external
command to use $WINDOWID instead of having to snprintf it again.  (The
same benefit will apply to the externalpipe patch.)  The xwinid function
is no longer needed.

Signed-off-by: Devin J. Pohly <djpohly@gmail.com>

win.h

@@ -19,12 +19,10 @@ void xinit(void);
 void xloadcols(void);
 int xsetcolorname(int, const char *);
 void xloadfonts(char *, double);
-void xsetenv(void);
 void xsettitle(char *);
 void xsetpointermotion(int);
 void xseturgency(int);
 void xunloadfonts(void);
 void xresize(int, int);
 void xselpaste(void);
-unsigned long xwinid(void);
 void xsetsel(char *, Time);