[Bug] v.db.connect advertises layer_number/layer_name syntax but gives warning

[wenzeslaus]

Describe the bug

The doc for v.db.connect says that layer can be in the formats layer number[/layer name], but using that produces a warning about an illegal filename and character </> not being allowed.

Doc:

       layer=string
           Layer number or name
           Format: layer number[/layer name]
           Default: 1

Message:

Illegal filename <1/geology>. Character </> not allowed.

To reproduce

$ grass ~/grassdata/nc_spm_08_grass7/PERMANENT/ --exec v.db.connect -c map=geology layer=1/geology
WARNING: Illegal filename <1/geology>. Character </> not allowed.
INTEGER|cat
DOUBLE PRECISION|onemap_pro
DOUBLE PRECISION|PERIMETER
INTEGER|GEOL250_
INTEGER|GEOL250_ID
CHARACTER|GEO_NAME
DOUBLE PRECISION|SHAPE_area
DOUBLE PRECISION|SHAPE_len

Expected behavior

I don't know what I expect to happen. What do you expect?

The tool-specific code clearly counts on the option of having number/name there as suggested by the documentation.

The tool not only documents number/name as a possible way, but also prints information in that format:

$ grass ~/grassdata/nc_spm_08_grass7/PERMANENT/ --exec v.db.connect -p map=geology 
Vector map <geology> is connected by:
layer <1/geology> table <geology> in database <.../grassdata/nc_spm_08_grass7/PERMANENT/sqlite/sqlite.db> through driver <sqlite> with key <cat>
$ grass ~/grassdata/nc_spm_08_grass7/PERMANENT/ --exec v.db.connect -g map=geology 
1/geology|geology|cat|.../grassdata/nc_spm_08_grass7/PERMANENT/sqlite/sqlite.db|sqlite

On the other hand, a warning is produced by the library.

When I use v.info and v.db.select, 1/geology (and also 1 and geology) are accepted without a warning. This may or may not be intentional given that when the conversion from string to a number is performed, atoi in C is likely used, and C atoi ignores whatever comes after the number (cplusplus.com):

The string can contain additional characters after those that form the integral number, which are ignored and have no effect on the behavior of this function.

Additional context

The internal struct field_info has:

int 	number
 	Layer number
 
char * 	name
 	Layer name (optional)

I found this when reviewing #6077.