osint.adb (Read_Default_Search_Dirs): correctly detect relative pathnames in UNIX and DOS style with drive letter.

	* osint.adb (Read_Default_Search_Dirs): correctly detect relative
        pathnames in UNIX and DOS style with drive letter.
	(Is_Relative): new routine.

	* osint.adb: Minor reformatting

	* osint.adb (Is_Relative): implementation using
        GNAT.OS_Lib.Is_Absolute_Path. Better fix for 8121-009.

From-SVN: r46503