ns_fseekchars - Seek in an open file for a string
This command seeks in a Tcl opened by Tcl channel fileId for the provided string searchstring. The search starts from the current position in the file. When the string is found, the command returns the position of the first character in the file and moves the file pointer to the start of this string. Therefore, the command can be used for multiple search operations in the file. If the string is not found, -1 is returned and the file position is at end-of-file.