report filename:line on runtime errors

[agordon]

On runtime errors (ending in invalid state at 'main.c:process()'),
print the input filename and current line number which triggered the
error.

• util.c:
  struct jq_util_input_state: add variables
  jq_util_input_init(): initialize new variables
  jq_util_input_read_more(): update current file/line number on
  fgets() calls.
  jq_util_input_get_position(): helper functions returning a JV_STRING
  containing the current file/line.
  strncpyz(): helper function for safe string copy
• main.c:
  process(): upon invalid result ('uncaught jq exception'), get the
  current input position and print it to stderr.
• jq.h: declare 'jq_input_get_position()'.

With this patch, runtime errors printed to stderr will contain the
filename and line of the offending input.

Examples:

With stdin and multiple lines:

$ printf '{"a":43}\n{"a":{"b":66}}\n' | ./jq '.a+1'
44
jq: error (at stdin:2): object and number cannot be added

With multiple files:

$ printf '{"a":43}' > 1.json
$ printf '{"a":"hello"}\n' > 2.json
$ printf '{"a":{"b":66}}\n' > 3.json
$ ./jq '[.a]|@tsv' 1.json 2.json 3.json
"43"
"hello"
jq: error (at 3.json:1): object is not valid in a csv row

With very long lines (spanning multiple fgets calls):

$ (  printf '{"a":43}\n' ;
     printf '{"a":{"b":[' ; seq 10000 | paste -d, -s | tr -d '\n' ;
     printf ']}}\n' ;
     printf '{"a":"hello"}\n' ) | ./jq '[.a] | @tsv'
"43"
jq: error (at stdin:2): object is not valid in a csv row
"hello"

With raw input:

$ seq 1000 | ./jq --raw-input 'select(.=="700") | . + 10'
jq: error (at stdin:700): string and number cannot be added

Caveat:
The reported line will be the last line of the (valid) parsed JSON data.
Example:

$ printf '{\n"a":\n"hello"\n\n\n}\n' | ./jq '.a+4'
jq: error (at stdin:6): string and number cannot be added

minor ugly hack:
The call the get the current filename/line in 'main.c' is hard-coded
to 'jq_util_input_get_position()' which somewhat bypasses the idea
of using an input callback (e.g. 'jq_set_input_cb()').
But since similar calls to 'jq_utl_input_XXXX' are also hard-coded in
'main.c', the input callback mechanism isn't really generatic at the moment.

[nicowilliams]

Thanks!