Check the -o path before looking for the mnemonic (closes #87)

age encrypt and age decrypt looked at the -o path only after the
mnemonic had been obtained, so a path that could never be written
still prompted for the mnemonic or ran the mnemonic command. Both now
refuse, before the key is derived, an -o path whose directory is
missing or is not a directory, or a symlink that points at nothing.
The check creates nothing; output still looks at the path when it
writes, so the file beside the target is made when it was before. The
README names the -o path among what is checked first.

Model: opus-5-5

README.md

@@ -124,10 +124,12 @@ refused with a message saying so. Keys are derived from the mnemonic's words
 joined by single spaces, whatever whitespace is around or between them, so one
 word per line, tabs or extra spaces give the same keys.
 
-The key index, `--words`, the `--to` recipients and the file `age encrypt` or
-`age decrypt` is given to read are checked before the mnemonic is looked for, so
-a run refused for one of them never prompts for the mnemonic or runs the
-mnemonic command.
+The key index, `--words`, the `--to` recipients, the file `age encrypt` or
+`age decrypt` is given to read and its `-o` path are checked before the mnemonic
+is looked for, so a run refused for one of them never prompts for the mnemonic
+or runs the mnemonic command. An `-o` path is refused at that point if its
+directory is missing or is not a directory, or if it is a symlink that points at
+nothing; anything else wrong with it shows only when the output is written.
 
 `KEYFUNC_MNEMONIC` and `KEYFUNC_MNEMONIC_COMMAND` are removed from the
 environment before the mnemonic command, the system `ssh` (`keyfunc ssh to`) and